1. Field of Invention
The present invention relates to digital-to-analog converters (DACs). More particularly, the present invention relates to high-resolution digital-to-analog converters.
2. Description of Related Art
Those skilled in the art know that various DAC architectures are available, such as a resistor string (R-string) DAC. FIG. 1 illustrates a schematic view of a conventional R-string DAC. An R-string DAC 100 includes a selector 122 and a plurality of resistors 112. The selector 122 includes a plurality of switching elements 126, such as MOS transistors, constructing a plurality of selecting lines 124 and switched by a digital input, such as an M-bit input word.
The resistors 112 are connected in series between a high reference voltage (VH) and a low reference voltage (VL). By the selector 122, the connecting nodes between the resistors 112 constitute tap points selectively switched to an output node in response to the digital input. The voltage of the tap point selectively switched to the output node is an inherently monotonic analog representation of the digital input.
For an M-bit digital-to-analog conversion, 2M tap point voltage levels are required, and so 2M+1 resistors are required for the R-string DAC. However, this number of resistors is too large to be practical for more than approximately eight bits. Moreover, 2M−1×M switching elements are also required. The number of switching elements is dramatically increased as the bits of the digital input are increased. However, when the number of switching elements is increased, the layout area of a chip occupied by the switching elements is also increased. It is very unfavorable for chip design and cost reduction.
For example, although the 10-bit R-string DAC deals with only 2 more bits than the 6-bit R-string DAC, the number of switching elements of the 10-bit R-string DAC is about 26.7 times greater than that of the 6-bit R-string DAC. In addition, the 10-bit R-string DAC requires 960 more transistors than the 6-bit R-string DAC. The number of switching elements decides the X-axis width of the chip, and the number of the resistors contributes the Y-axis height of the chip. That is, the bit increasing of the digital input critically affects the chip size.